Wireless communications systems such as those operating in accordance with CDMA2000 DO standards, for example, are optimized for handling high-speed data applications on the reverse link from the mobile terminals (referred to as Access Terminals [ATs] in CDMA2000 DO terminology) to a base station (referred to as a Base Transceiver Station [BTS] in CDMA2000 DO terminology). Unlike other traffic, such as voice, which is continuous in nature, the high-speed data transmitted by an AT is generally discontinuous and bursty. Due to the bursty nature of data calls, a larger number of data calls can be admitted into the system than could be if all ATs were always transmitting simultaneously. With so many data calls involved in established data connections, there are times when the radio link cannot handle the number of ATs actually simultaneously transmitting on the RL at the rate they desire.
A normal Reverse-link Overload Control (ROC) is used to control radio link loading through adjustment of the transmission rates of the ATs that are transmitting on the reverse link. The ROC is based on a measure at the BTS of a comparison of measured the Rise Over Thermal (ROT) against a threshold, where the ROT is a measure of the total received power on the reverse link over a thermal noise floor, and is an indicator of the total traffic on the reverse link. The ROT is continually measured and compared against the threshold. If the ROT is greater than the threshold, then the BTS sets a Reverse Activity Bit (RAB) to “1” and broadcasts it downlink to all active and connected ATs. A receiving AT then reduces its transmission rate, but not below the minimum rate required by the application that it is running. If the measured ROT is less than the threshold, the RAB is set to “0”. A receiving AT then performs a predetermined calculation to determine whether or not and how to increase its transmission rate. Typically, the RAB is transmitted from the BTS to all ATs on the Reverse Activity (RA) channel on the forward link each time slot in a frame, where the number of slots per frame is 16 in the exemplary CDMA2000 DO system and the length of a frame is equal to 26.67 ms. Generally the RAB received by the ATs within a sector will continuously vary between “1” and “0”, causing the transmission rates of the connected and active ATs to decrease and increase, while still enabling the ATs to meet their quality requirements (e.g., Frame Error Rate [FER]).
A congestion overload (CO) condition occurs when too many ATs are simultaneously transmitting on the reverse link even after all the receiving ATs have reduced their transmission rate to their minimum allowed values. When such a CO condition occurs, the performance levels of all the ATs decrease so that they are unable to meet their required minimum FER. Meanwhile, during congestion overload, new calls continue to be admitted to the system further exacerbating the situation. After a period of time, with continued poor FER, calls will be automatically dropped. Dropping calls creates its own deleterious side effects as network resources are wasted in reconnecting the dropped calls. Furthermore, reconnection requests sent by the dropped ATs create additional interference on the already congested system.
An effective mechanism for handling a congestion overload condition due to too much traffic on the reverse link is thus required.